If you have that amount of Bitcoins worth of 15m and you are the only one supporting yourself, why not retire at 44 years old? Since that amount is huge amount, I think a person can live with $44k annually, depending on where you live. The $300,000 is too much for an annual budget; I am not sure how you spend such a large amount unless you live with a family.
Well, I don’t know how easy it seems for you guys to call all these heavy figures because if these mentioned figures are converted to my local currency, then meh I don’t think it will be wise for you to still be working for someone else but rather should be in a more better position to create opportunities for others.
I don’t know how other country systems operate but for me, I think retiring early with such amount of money is the best but mind you, retirement doesn’t mean, you have to stop working or stop putting your brain at work but eBay it simply means is that, with such about of money($15m) one should be able to create opportunities that enables him be in charge of his time while creating a future and a comfortable life during old age.
You don't have to liquidate all your holdings to live off it for the rest of life. Liquidate some, get the life you want, and let the rest grow.
In lame man's words, invest and invest until you don’t have to check price while shopping and anyone who retires only to live off their savings is heading for something not nice